3 Answers
3

For various purposes: sometimes for compatibility between naming schemes, sometimes it is necessary for the working environment - as in the example of /dev/stdin. This does not point statically to /dev/pts/2 or any other - just switch to another terminal and you'll see. /dev/stdin is the standard input of your current terminal session. That is also an example why it needs to be a symlink.

"standard input of your current terminal session" is a bit ambiguous. /dev/stdin refers to the standard input of the process that will open it. Everything in /proc/$pid is process dependent data, and /proc/self is a kind of magic symlink pointing to the process' own data.
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Stéphane GimenezAug 11 '11 at 1:09

Almost all the files under /dev are device files. Whereas reading and writing to a regular file stores data on a disk or other filesystem, accessing a device file communicates with a driver in the kernel, which generally in turn communicates with a piece of hardware (a hardware device, hence the name).

There are two types of device files: block devices (indicated by b as the first character in the output of ls -l), and character devices (indicated by c). The distinction between block and character devices is not completely universal. Block devices are things like disks, which behave like large, fixed-size files: if you write a byte at a certain offset, and later read from the device at that offset, you get that byte back. Character devices are just about anything else, where writing a byte has some immediate effect (e.g. it's emitted on a serial line) and reading a byte also has some immediate effect (e.g. it's read from the serial port).

The meaning of a device file is determined by its number, not by its name (the name matters to applications, but not to the kernel). The number is actually two numbers: the major number indicates which driver is responsible for this device, and the minor number allows a driver to drive several devices¹. These numbers appear in the ls -l listing, where you would normally find the file size. E.g. brw-rw---- 1 root disk 8, 0 Jul 12 15:54 /dev/sda → this device is major 8, minor 0.

Some device files under /dev don't correspond to hardware devices. One that exists on every unix system is /dev/null; writing from it has no effect, and reading from it never returns any data. It's often convenient in shell scripts, when you want to ignore the output from a command (>/dev/null) or run a command with no input (</dev/null). Other common examples are /dev/zero (which returns null bytes ad infinitum) /dev/urandom (which returns random bytes ad infinitum).

A few device files have a meaning that depends on the process that accesses it. For example, /dev/stdin designates the standard input of the current process; opening from has the same effect as duplicating standard input². Somewhat similarly, /dev/tty designates the terminal to which the process is connected. Under Linux, nowadays, /dev/stdin and friends are not implemented as character devices, but instead as symbolic links to a more general mechanism that allows every file descriptor to be referenced (as opposed to only 0, 1 and 2 under the traditional method); for example /dev/stdin is a symbolic link to /proc/self/fd/0. See How does /dev/fd relate to /proc/self/fd/?.

You'll find a number of symbolic links under /dev. This can occur for historical reasons: a device file was moved from one name to another, but some applications still use the old name. For example, /dev/scd0 is a symbolic link to /dev/sr0 under Linux; both designate the first CD device. Another reason for symbolic links is organization: under Linux, you'll find your hard disks and partitions in several places: /dev/sda and /dev/sda1 and friends (each disk designated by an arbitrary letter, and partitions according to the partition layout), /dev/disk/by-id/* (disks designated by a unique serial number), /dev/disk/by-label/* (partitions with a filesystem, designated by a human-chosen label); and more. Symbolic links are also used when a generic device name could be one of several; for example /dev/dvd might be a symbolic link to /dev/sr0, or it might be a link to /dev/sr1 if you have two CD readers and the second one is to be the default DVD reader.

Finally, there are a few other files that you might find under /dev, for traditional reasons. You won't find the same on every system. On most unices, /dev/log is a socket that programs use to emit log messages. /dev/MAKEDEV is a script that creates entries in /dev. On modern Linux systems, entries in /dev/ are created automatically by udev, obsoleting MAKEDEV.

¹ This is actually no longer true under Linux, but this detail only matters to device driver writers.
² Duplicating a file descriptor means creating a new file descriptor that accesses the same file as the first. The corresponding system call is dup: open("/dev/stdin", flags) is the same as dup(0), except that the new descriptor may be created with different access modes.

Thanks! By "The meaning of a device file is determined by its number", do you mean its file descriptor?
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TimAug 11 '11 at 2:54

@Tim: No, the numbers appear in ls -l listing where you normally find the file size, before the date, e.g. brw-rw---- 1 root disk 8, 0 Jul 12 15:54 /dev/sda → this device is major 8, minor 0. Device numbers don't come up often in practice, I just mentioned them to say what makes a device a device (most importantly, it's not the file name). A file descriptor number only has a meaning in a particular process.
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GillesAug 11 '11 at 7:35

For your first question, they are not file descriptors, they are device files. (a.k.a. "dev nodes")

Those files are bound with the driver that is handling the device using major and minor numbers. (For instance, "136, 2" in your ls output refers to the device driver bound to major number 136, and specifies device #2 handled by that driver.)

The first letter of output of ls -l is type of the device in case of device files. If it is 'c' it is a character device, or if it is 'b', it is a block device.